Don't pass error from syscallenter() to syscallret().

[jhb]

Summary

syscallret() doesn't use error anymore. Fix a few other places to permit
removing the return value from syscallenter() entirely.

• Remove a duplicated assertion from arm's syscall().
• Use td_errno for amd64_syscall_ret_flush_l1d.

Test Plan

• compiled amd64 and i386, still need to do a full tinderbox

[kib]

Thanks.

sys/arm/arm/syscall.c

146

Remove this blank line as well.